Electric wave system



Aprii 22, 1941, P. J. WALSH. 2,233,986

ELECTRIC WAVE SYSTEM Filed Dec. 5, 1939 INVENTOR Patented A r. 22, 1941 UNITED STATES PATENT OFFICE 2,238,986 I ELECTRIC WAVE SYSTEM" Philip John Walsh, San Francisco, Calif. Application December 5, 1939, serial No. 307,629 3 Claims. (01. 250-40) This invention relates to tuning systems, and especially to radio receiving systems.

In the superheterodyne type of radio receiver, it is well known that, for each local oscillator frequency there are two possible carrier 'frequencies which will give the same difference frequency. In order to prevent simultaneous reception of two stations in this way it has been found necessary to employ a tuned radio-frequency input amplifier ahead of the first detector. This leads to all sorts of complicated switching arrangements in receivers designed to operate at standard broadcast and short wave frequencies.

It is accordingly one of the objects of my invention to overcome these difliculties and to provide a simple scheme for of selectivity.

In radio receivers, transmitters and other devices where a number of tuned circuits are adjusted simultaneously to different frequencies by means of a single control knob, the individual tuned circuits must be broadly tuned. This is due to the fact that sharply tuned circuits are so critical of adjustment that gang control to a sinobtaining a high degree gle tuning control knob is practically impossible.

In receivers designed to receive telephone signals sharply tuned high efiiciency circuits can not be employed for this reason and also due to the fact that a wide band of frequencies must be amplified without excessive side-band cutting and distortion. Since the broadly tuned circuit has far less frequency selective action than the sharply tuned circuit, more amplifier stages and tuned circuits are required to obtain the same degree of selectivity (ability to keep out unde- I1 sired signals), as would be needed tuned circuits could be employed. g

It is one of the objects of my invention to overcome these difficulties and to provide a simple scheme, without the use of amplifiers or other tuned circuits, to increase the overall frequency selective action of a broadly tuned circuit.

My invention possesses many other advantages and has other objects which may be more easily if sharply apparent from a consideration of one embodiment of my invention. For this purpose, I have shown a few forms in the drawing accompanying and forming part of the present specification. I shall now proceed to describe these forms in detail, which illustrate the general principles of my invention; but it is to be understood that this detailed description is not to be taken in a limiting sense, since the scope of my invention is best defined by the appended claims. 1

Referring to the drawing:

" modification.

nection 1 to the center tap l2 of the high impedance choke coil 8 where the current divides, part going through the left hand half of coil 8, connection H and through resistor 9, tap In, connection 20 to connection 2| and terminal 4; the other'part going through the right hand half of coil 8, connection 22, tap 23, coil 24, connection to connection 2| and terminal 4. Since the signalling impulses are alternating currents they flow in both directions through the circuit just traced. At one instant from the antenna l to terminal 3 and through this circuit to terminal 4 and ground 2, and at another instant from ground 2 to terminal 4 and through this circuit to terminal 3 and antenna l.

The circuit 21 comprising the inductance coil 24 and the variable condenser 26 is a parallel resonant circuit and its properties are well understood. When electrical impulses are impressed on the circuit 2 1 between tap 23 and terminal 4, at very low-frequencies the coil 24 draws a large lagging current while the leading current of the condenser 26 is small, resulting in a large lagging current through tap 23 and connection 25 and a low lagging circuit impedance. At high 1 frequencies the coil 24 has a high reactance compared with the condenser 26, resulting in a large leading currentthrough tap 23 and connection 25 and a correspondingly low circuit impedance that is'leading in phase. In between these two extremes there is a frequency at which the lagging current through the coil 24 and the leading current through the condenser 26 are equal, and

being degrees out of phase neutralize, leaving only a small resultant in-phase current flowing through the external tap 23 and connection 25., At this resonant frequency the impedance of the parallel circuit 2! will be a very high resistance.

*Asthe description proceeds I will explain how these properties of the tuned circuit 21 are utilized to, automatically control the action of the extremely high impedance choke coil 8 so that it 3 acts asalowresistance to desired signals and simultaneously as a high impedance choke coil to all undesired signals.

The upper end of the coil 24 is connected to the control electrode 28 of the electronic emission device 29 by means of conductor 38, and the lower end of coil 24 is connected to the cathode 3| thereof by means of conductors 25, 2| and 32. The bias battery 33 maintains a negative bias voltage on the grid 28, the filament battery 34 furnishes current to heat the cathode 3|, and the plate battery 35 maintains the plate 36 and screen grid 31 at a potential positive relative to the cathode 3|. Condenser 42 keeps the screen electrode 31 at ground potential so far as alterhating currents are concerned. Sources of power other than these batteries can be employed, as well as other types of electronic emission devices, such as the so-called detector-oscillator tube, as used in superheterodyne radio receivers. .Since the operation of such devices is now well understood further detailing thereof is unessential, except to point out that the electrical impulses which appear across the circuit 21 are applied to the grid 28 and cathode 3| and reappear as amplified impulses across the terminals 48 and 4|, where they can be applied to any kind of utilization or work circuit.

Now let us assume that the circuit 21 is tuned by the variable condenser 28 to be resonant at the frequency of a desired signal which is being picked up by the antenna I. Also let us assume that the tap H) on the resistor 9 is adjusted so that its resistance is'equal to the resistance of circuit 21 as measured at this resonant frequency between tap 23 and terminal 4. Under these circumstances there are two parallel paths of equal .and substantially pure resistance to alternating currents impressed across the terminals 3 and 4 by the antenna I and ground 2. When the current flows from terminal 3, through connection 1 to the center tap l2 on the coil 8, half goes through the left hand half of coil 8, connection I resistor 9, tap l8 and connections 28 and 2| to terminal 4. The other half flows fromthe center tap I2 through the right hand half of coil 8, in the opposite direction, thence by Way of connection 22, circuit 21, and connections 25 and. 2| to terminal 4. During the other half of the cycle the current fiOWs from terminal 4 to terminal 3 in the opposite direction through the two parallel circuits just traced. Since current flows in one direction in the left hand half of coil 8, and an equal current flows in the opposite direction through the right hand half of coil 8 coil 8 is substantially a low non-inductive resistance.

Thus the esired signal impulses flow readily through the coil 8 with little opposition, to the circuit 21 and are amplified by the electronic emission device 29 and appear across the terminals 48 and 4| as already described.

Now let us assume that the antenna I is strongly affected by an undesired signal. Since the circuit 21 is not tuned to this signal it does not act as a high resistance. Its impedance (as measured between tap 23 and terminal 4) is now very much less than its resonant impedance and the current is out of 'phase with the applied voltbeen described. Thus the balance beage as has tween circuit 21 and resistor 8 is destroyed causing coil 8 to act as an extremely high impedance '1 choke coil which reduces the current flowing therethroug h to a value that is vanishinglysmall. Thus the undesired impulses have practically no eiiect onthe tube 28.

It is seen that the circuit made up of tap 23,

connection 22, coil 8, connection ll, resistor 8, tap l8, connection 28, and connections 2| and 25 back to circuit 21, does not act as a load across circuit 21 due to the high impedance of choke coil 8. Therefore, any feed-back through the tube 29 into the circuit 21 (or oscillator feed-back in a superheterodyne), cannot cause currents to flow in the antenna circuit where they would radiate interference impulses to other receivers.

In Fig. 2, I show another form of my invention the operation of which is similar to the form shown in Fig. l. The choke coil 58 operates in the same manner as the choke coil 8 of Fig. l. The tuned circuit 5| corresponds to the tuned circuit 21 of Fig. 1. In this form of the inven tion the source of alternating current such as the antenna 59 and ground 68 connected to the input terminals 57 and 58 respectively, impresses signalling impulses on the two parallel paths made up of connection 6|, tap 82, half of coil 58, connection 63, coil 53, resistor 52, tap 64, connections 65 and 66 to terminal 58; the other path comprising half of coil 58, connection 61, coil 54 and conductor 66 to terminal 58. The coil 53 serves as a balance coil. The tuned circuit 5| comprising the secondary coil 55 and condenser 56 is magnetically coupled to the primary coil 54. Since the action of this coupled circuit is well understood further detailing thereof is unessential, except to point out that the impedance which the tuned secondary circuit 5| couples into the primary 54 has all the essential characteristics of the impedance of this same tuned secondary circuit when acting as a parallel resonant circuit. Therefore, for a desired signal to which the circuit 5| is tuned, the impedance coupled back into the coil 54 is a resistance equal in value to the adjusted resistance of resistor 52 in order to reduce the high impedance choke coil 58 to a low non-inductive resistance, as has been described. Thus the desired signal currents flow through coil 54 and induce currents in the secondary coil 55 which. producea resonant rise in voltage across circuit 5| to which the output terminals 88 and 69 are connected. For undesired signals the actions are substantially the same as set forth in the description of Fig. 1. That is, circuit 5| does not couple back to coil 54 an impedance equal to the resistor 52 and therefore coil 58 is a high impedance choke coil which substantially prevents the flow of current through coil 54.

I claim:

1. A selective tuning circuit comprising an inductance comprising two mutually inductive equal sections, a parallel-resonant circuit connected to one end of said inductance, a non resonant circuit connected to the other end of said inductance and to said parallel-resonant circuit, input terminals at the junctions of said sections and of said resonant and non-resonant circuits respectively, and output terminals across said parallel-resonant circuit.

2. The method of selective tuning with a parallel-resonant circuit and a resistive circuit which comprises connecting said circuits as parallel branches, closely coupling said branches inductively, balancing the self-inductance of said circuits and adjusting said parallel circuit to resonance at the selected frequency, and adjusting said resistive circuit to substantially the same impedance as said parallel circuit, the inductive coupling of said branches being in such sense that the mutually induced voltages due to equal flow in said branches tend to oppose the self-induced voltages, whereby the apparent impedance aeeaeec 3 in series with said resonant circuit increases as the frequency applied thereto departs from said selected frequency.

3. A selective tuning system comprising a circuit of two parallel branches, one of said branches having a predominantly resistive impedance-frequency characteristic, the other of said circuits having a parallel-resonant impedance characteristic, means for balancing the impedance of said circuits to substantial equality at 10 the frequency to be selected, and an inductor comprising two closely coupled sections connecting said branches, one of said sections being included in each branch, and said sections comprising eifectively a continuous Winding wherein the voltages induced mutually by currents dividing between said branches are opposed to the self-inductive voltages in said branches.

PHILIP JOHN WALSH. 

